Conventional rotary encoders are constructed as shown, for example, in FIGS. 8 and 9. FIG. 8 is a front view with the configuration of the rotary-transducer base member and the bearing area, and FIG. 9 is a cross-sectional view taken along line A-A of FIG. 8, the view of FIG. 8 having been supplemented with further components of the rotary encoder. To make it easier to understand the arrangement, base-plate supports 1a, which are not actually located at this position in the cross-sectional view, are also indicated in FIG. 9.
The rotary encoder has a rotary-transducer base member 1, bearings 2 disposed on rotary-transducer base member 1, and a hollow shaft 3 rotatably supported by these bearings 2. The shaft of an object to be measured is inserted into the hollow region of the end of shaft 3 to the left in FIG. 9, making it possible to couple shaft 3 of the rotary encoder to the rotation of the object to be measured. At the right end of shaft 3, a code disk 4 is mounted which rotates coupled with the rotation of the shaft. Because of the rotation, the annular measuring graduation on code disk 4 rotates, which is made up of alternating slots and opaque areas.
Mounted on supports 1a of rotary-transducer base member 1 is a base plate 5 or board, which corresponds to the shape of rotary-transducer base member 1. At a specific location of base plate 5, a detector element 6 is disposed, which is provided with a fixed scanning graduation 7 situated in front of it. In addition, further electronic components and circuits for processing the electrical signals from detector element 6 are formed and arranged on the base plate. A light source 8 is disposed in a recess or opening 1b formed on rotary-transducer base member 1, the respective positions being coordinated such that light source 8 is opposite detector element 6, with code disk 4 interposing. The light from light source 8 passes through the transparent slots formed on code disk 4 and strikes detector element 6, before which fixed scanning graduation 7 is situated. By detecting the light which passes through the measuring graduation of the code disk, it is possible to determine the rotational position of code disk 4, that is, the rotational position of the object to be measured.
FIGS. 10 and 11 show the construction of another conventional rotary encoder, FIG. 10 being a front view of the configuration of the rotary-transducer base member and the bearing area. FIG. 11 a cross-sectional view taken along line A-A from FIG. 10, in which the view of FIG. 10 is supplemented with further components of the rotary encoder. In this example, for reasons of weight, a thin region 1d is formed between periphery 1g of rotary-transducer base member 1 and a bearing fixation region 1e, and a rib 1c is formed for reinforcement between supports 1a and bearing fixation region 1e. The further components correspond to those of FIGS. 8 and 9, which is why identical components are provided with identical reference numerals, and their explanation is omitted.
The following problems exist in connection with conventional rotary encoders constructed in this manner. As mentioned above, shaft 3 is disposed via bearings 2 on rotary-transducer base member 1. While rotary-transducer base member 1 is normally made of a metal that is light and easy to work such as aluminum or something similar, iron is often selected as material for bearings 3. Therefore, if there is a change of temperature in the rotary-transducer surroundings, an unwanted mechanical stress or load of the rotary encoder results because of the different coefficients of thermal expansion owing to the different properties of the two materials. Due to the stress on the part of rotary-transducer base member 1, bearings 2 can deform, which may result in rotational irregularities or torque fluctuations of shaft 3.
This may even have an effect on the object to be measured. Therefore, the danger exists of disadvantageously influencing the exact measurement of the rotational speed and the rotational position, i.e., disadvantageously influencing the positioning accuracy. If the connection to the object to be measured is accomplished via a belt or something similar, the belt will slip and measuring errors will occur.
Optical rotary encoders having a design of this kind are described, for instance, in Japanese Published Patent Application No. 2001-27551. However, the problem mentioned in connection with the influence of different coefficients of thermal expansion of the rotary-transducer base member and of the bearings is not mentioned.